The spatial patterning or architecture of any given tree is ultimately a consequence of numerous developmental, genetic, and environmental factors (Barthélémy D. and Caraglio Y., 2007, Ann Bot. 99, 375-407). Over the last 40 years, the study of tree architecture has intensified as it is a critical parameter for both the management and aesthetics of our agriculture, forests, and residential landscapes. Two features that prominently contribute to tree architectural differences include the pattern of branching and the angle of branch growth (Tomlinson, P. B., 1978, London: Cambridge University Press, 197-202). Combined, these factors control the number of branches produced, their spacing, and their directional orientations.
Branch growth angle is not uniform and subject to substantial variation within any given tree. Yet in a broad sense, branch angle has been used to classify trees into architectural types that are influenced by various tropisms. These include pillar or columnar forms that have narrow branch angles, spreading types with wide branch angles, and weeping in which shoots grow downwards. Unlike the apical shoot, axillary shoots are not subject to strict gravitropic or phototropic control, allowing them to grow in a variety of directions irrespective of the gravity and light vectors. Most familiar tree canopy shapes display a phenomenon whereby shoots closer to the apical meristem grow more vertical while those lower in the canopy tend to grow more horizontally.
Optimizing tree architecture to maximize productivity and simplify management is a chief goal of numerous tree crop industries. Thus tree architecture has been long studied with regard to horticultural practices associated with orchard and plantation forestry management. Tree growth responses to various types of pruning, hormone treatments, fertilizer applications, and effects of rootstock-scion interactions are well established. Architectural tree types suited for high density production systems and/or improved mechanization offer great promise for improving tree-based agricultural systems. In this regard, vertical or columnar tree forms are being investigated due to their erect axillary branch angles and reduced canopy diameter (Kelsey, D. F. and Brown, S. K., 1992, Fruit Var. J. 46, 83-87; Scorza et al., 1989, J. Am. Soc. Hortic. Sci. 114, 98-100). Considering the importance of canopy spatial patterning to the evolution and niche exploitation of land plants, little is known of the genes underlying the genetic basis of these traits.
Peach trees with extreme vertical branching resulting in a fastigiated tree shape have been developed for use in high density production systems (Miller and Scorza, 2010, J. Amer. Pomological Soc. 64, 199-217). This trait was initially referred to as “broomy” (br) (Yamazaki et al., 1987, New broomy flowering peach cultivars Terutebeni, Terutemomo, and Teruteshiro. Bulletin of the Kanagawa Horticultural Experiment Station, No. 34) and was later designated “pillar” inasmuch as it was shown to be incompletely dominant as heterozygous individuals have intermediate branch angles referred to as “upright” (Scorza et al., 1989, J. Am. Soc. Hortic. Sci. 114, 98-100; Tworkoski and Scorza, 2001, J. Amer. Hort. Sci. 126, 785-790; and Scorza et al., 2002, J. Amer. Soc. Hort. Sci. 127(2), 254-261). Axillary shoots in vertical trees tend to grow vertically regardless of their canopy position. Given the growing need to control canopy special patterns in trees, especially trees planted in nursery and orchards settings, there is a need to develop breeding and identification techniques to identify the genes responsible for different growth habits in tree varieties.